1. Field of the Invention
This invention relates to noncontact testing of integrated circuits, and more particularly relates to full-line dynamic AC testing of integrated circuits at very high speed and resolution using laser induced photoemission modulated by instantaneous circuit operation voltages.
2. Description of the Prior Art
A copending U.S. patent specification by Beha, Dreyfus and Rubloff discloses techniques for using photoemission induced by a pulsed ultraviolet laser to measure the voltage at specific points on a VLSI or VHSIC chip in real time with every high time resolution (about 10 picoseconds or less). The measured photocurrent reflects directly the voltage of the point on the chip being tested because this voltage affects the acceleration or retardation field sensed by the emitted electron. Specific laser systems capable of pulse widths in the picosecond range (or less) were proposed, and embodiments given, so that the actual AC switching waveform at a single point could be measured with picosecond time resolution, i.e., fast enough for advanced high speed circuits. Although such laser systems, already available in state-of-the-art laboratories, can be used for sequential measurements of this kind over the entire chip by scanning the laser beam, the power of such laser systems (operating in the picosecond range) today is not sufficient to accomplish simultaneous measurement of many points on the chip--i.e., for full-chip testing of a two-dimensional array of points as desired for manufacturing and other applications.
The following publications are representative of the prior art: U.S. Pat. No. 3,124,790, Kuehler, APPARATUS FOR READING MAGNETIC OR ELECTROSTATIC BITS, IBM, Mar. 10, 1964. Kuehler shows an electron beam scan of a pattern of recorded data, with varying deflection depending upon bit value.
U.S. Pat. No. 4,220,854, Feuerbaum, METHOD FOR THE CONTACTLESS MEASUREMENT OF THE POTENTIAL WAVEFORM IN AN ELECTRONIC COMPONENT AND APPARATUS FOR IMPLEMENTING THE METHOD, Sept. 2, 1980. Feuerbaum shows a phase comparison technique for using an e-beam scanner to measure the potential waveform in an electronic component.
U.S. Pat. No. 4,301,409, Miller et al, SOLAR CELL ANOMALY DETECTION METHOD AND APPARATUS, Nov. 17, 1981. Miller et al shows a technique for using a light scanner (dual laser or broad-spectrum) to test a solar cell for anomalies by biasing the entire cell with light of deep penetration wavelength while focussing a small light spot on the surface.
The prior art does not teach nor suggest the invention, which is column scanning across an array detector which in turn is visually responsive to row scanning of an emission pattern produced by an integrated circuit chip-to-test in response to dynamics of test operation and a laser beam focused to a line image, to provide good space-time resolution by crosspoint selection of threshold voltages at the array detector so as to provide an array output at each differential instant of time during the chip test-time.